DN262 - SOT-23 1kHz to 30MHz Oscillator with Single Resistor Frequency Set

SOT-23 1kHz to 30MHz Oscillator with
Single Resistor Frequency Set – Design Note 262
Andy Crofts
⎧100, DIV=V +
⎪
⎛ 10kΩ ⎞
ƒ OSC = 10MHz • ⎜
,N = ⎨10, DIV = Open
⎟
⎝ N •RSET ⎠
⎪1, DIV=GND
⎩
This simple and accurate relationship is achieved with
a proprietary design that linearizes the resistanceto-frequency conversion, eliminating errors such as
oscillator propagation delay.
Tiny Circuit, Big Performance
As shown in Figure 1, a complete oscillator requires only
an LTC1799, a frequency-setting resistor and a bypass
capacitor. The circuit’s small component count and the
LTC1799’s diminutive SOT-23 package add up to big
savings in PCB space when compared to oscillators
built from crystals, ceramic resonators, 555 timers or
discrete components.
5V
10MHz ±1.6%* (27°C)
5
OUT
LTC1799
2
GND
1
RSET
10k
0.1%
C1
0.1μF
3
V+
SET
DIV
4
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*INCLUDING ERROR CONTRIBUTION FROM RESISTOR
Figure 1. Complete Oscillator Solution
07/01/262_conv
You don’t pay a performance penalty for this miniaturization. The LTC1799 has a guaranteed frequency
accuracy of ±1.5% (±0.5% typical) at room temperature.
This spec applies over the entire 2.7V to 5.5V supply
range, made possible by the stingy 0.05%/V typical
drift over supply voltage.
The accuracy remains tight over temperature as well. The
LTC1799C has a typical temperature drift of ±0.004%/°C,
with guaranteed accuracy of ±2% over 0°C to 70°C
(LTC1799I guaranteed accuracy is ±2.5% over –40°C
to 85°C). Figure 2 shows the frequency output of the
circuit in Figure 1 over the industrial temperature range
for three typical parts.
10.04
PART #1
10.02
FREQUENCY (MHz)
The LTC ®1799 resistor-programmed oscillator eliminates the hassle in designing an accurate square-wave
frequency reference. A single resistor (RSET ) connected
between the power supply and an input pin (SET) programs the frequency of a master oscillator to a value
between 100kHz and 30MHz. An internal clock divider
is programmed using a three-state input pin (DIV) to
divide the master oscillator frequency by 1, 10 or 100
before driving the output. This extends the lower limit
to 1kHz for a total range of 1kHz to 30MHz. The output
frequency is linearly related to RSET, as defined by the
frequency-setting equation:
PART #2
10.00
PART #3
9.98
9.96
9.94
–40
–20
40
0
60
20
TEMPERATURE (°C)
80
100
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Figure 2. Frequency vs Temperature for Figure 1’s Circuit
Due to its low sensitivity to supply and temperature
variation, the LTC1799 has abilities that no other oscillator can match. Replacing RSET with a potentiometer
allows the output frequency to be “tuned” after the
circuit is completed. Once set, the LTC1799 will accurately maintain the desired frequency over all operating
conditions. Crystals and ceramic resonators cannot
be adjusted in this manner; 555 timers and other RC
oscillators do not have this level of stability.
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The LTC1799 is immune to vibration and acceleration
forces, another problem that plagues crystal oscillators. And its 1mA typical supply current (2.4mA max at
10MHz, 5V supply) is very efficient when compared to
the 10mA to 30mA many crystal oscillators consume.
Two-Step Design Process
The LTC1799 combines infinite frequency selectivity (limited only by resistor selection) with incredible
ease-of-use. The external resistor RSET determines
the frequency of the master oscillator within a 100kHz
to 30MHz range. The three-state DIV pin determines
whether the master oscillator signal is passed directly
to the output, or first divided by 10 or 100. The design
process is simple:
1. Use Table 1 to determine the proper divider setting.
Table 1. Frequency Range vs Divider Setting
DIVIDER SETTING
DIV (Pin 4)
CONNECTION
FREQUENCY
RANGE
÷1 (N = 1)
GND (Pin 2)
>500kHz*
÷10 (N = 10)
Floating
50kHz to 1MHz
÷100 (N = 100)
VCC (Pin 1)
≤100kHz
* At frequencies above 10MHz (RSET < 10k), the LTC1799 may
suffer reduced accuracy on supplies less than 4V.
2. With N known, calculate the best value for RSET
using the equation:
⎛ 10MHz ⎞
RSET = 10k • ⎜
⎝ N • ƒ OSC ⎟⎠
It’s that simple! Of course, since the LTC1799 converts
resistance into frequency, any errors in the value of RSET
(due to resistor tolerance or nonideal choice of resistor
value) reduce the frequency accuracy. Therefore, 1% or
0.1% resistors are recommended for best performance.
Application: Temperature-to-Frequency Converter
The most straightforward application for the LTC1799 is
as a constant-frequency reference. But its resistance-toData Sheet Download
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Linear Technology Corporation
frequency conversion architecture allows for a variety of
applications. Figure 3 shows a temperature-to-frequency
converter that is built by simply replacing RSET with a
thermistor. The YSI 44011 has a resistance of 100k at
25°C, 333k at 0°C and 16.3k at 70°C, a span that fits
nicely into the LTC1799’s permitted range for RSET.
With its low tempco and high linearity, the LTC1799
adds less than 0.5°C of error over the commercial
temperature range. Figure 4 plots the output frequency
vs temperature.
5V
1
C1
0.1μF
RT
100k
THERMISTOR
RT: YSI 44011
V+
OUT
LTC1799
2
GND
5
3
4
SET
DIV
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Figure 3. Temperature-to-Frequency Converter
1200
1000
FREQUENCY (kHz)
Fast Start-Up Time
One common problem designers encounter with crystal
oscillators is the long start-up time before the circuit is
oscillating at its final frequency. At MHz frequencies, this
start-up time is typically 10ms. At frequencies below
100kHz, a crystal oscillator may take up to a second
to start up. The LTC1799 takes less than 1ms to settle
within 1% of any frequency between 5kHz and 30MHz.
800
600
400
200
0
–20
0
20
40
60
TEMPERATURE (°C)
80
100
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Figure 4. Output Frequency vs Temperature
for Figure 3’s Circuit
Conclusion
The LTC1799 is a tiny, accurate, easy-to-use oscillator
that is programmed by a single resistor. With a typical accuracy of better than 0.5% and low temperature
and supply sensitivity, the LTC1799’s performance
approaches that of crystal oscillators and ceramic
resonators and yet requires far less PCB space. The
resistance-to-frequency conversion architecture allows
infinite resolution and design simplicity. The result is
a square-wave oscillator with an unprecedented combination of ease-of-use and precision in a tiny SOT-23
package.
For applications help,
call (408) 432-1900
dn262f_conv LT/TP 0701 345K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2001
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